Re: net/ipv4: use-after-free in ipv4_datagram_support_cmsg

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Wed, Apr 12, 2017 at 4:47 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Wed, 2017-04-12 at 13:07 -0700, Cong Wang wrote:
>> On Wed, Apr 12, 2017 at 8:39 AM, Willem de Bruijn
>> <willemdebruijn.kernel@gmail.com> wrote:
>> > ===================
>> >> BUG: KASAN: use-after-free in ipv4_datagram_support_cmsg
>> >> net/ipv4/ip_sockglue.c:500 [inline] at addr ffff880059be0128
>> >
>> > Thanks for the report. This is accessing skb->dev from within recvmsg() at line
>> >
>> >         info->ipi_ifindex = skb->dev->ifindex;
>> >
>> > Introduced in 829ae9d61165 ("net-timestamp: allow reading recv cmsg on
>> > errqueue with origin tstamp"). At this time the device may indeed have
>> > gone away. I'm having a look at a way to read this in the receive BH
>> > and store the ifindex.
>>
>> Why not use skb_iif?

This code is called from the error path for transmit timestamps.

We can make use of the fact that SKB_EXT_ERR used on enqueue has iif as
the first field in its control block. This also holds for the PKTINFO_SKB_CB
struct to which skb->cb is cast on dequeue when it copies pktinfo to userspace.
So if set on enqueue in __skb_complete_tx_timestamp, no conversion operation
is even needed on dequeue, let alone the currently buggy line that touches
skb->dev.

This iif cast was added for this purpose in the receive path in 0b922b7a829c
("net: original ingress device index in PKTINFO").

The device pointer is valid on enqueue for all paths called from device drivers,
as well as from dev_queue_xmit for SCM_TSTAMP_SCHED generation in
__dev_queue_xmit. The exception is SCM_TSTAMP_ACK generation, but
there skb->dev is NULL.

The v6 path does need a conversion, but already does this in
ip6_datagram_recv_common_ctl. There, too, we can remove the buggy
logic to set it from skb->dev->ifindex in ip6_datagram_support_cmsg.

I will send a patch.